In heavy duty drilling tools, such as rock drilling tools, the drill bit has a working end shaped to bore holes and/or remove material and an opposite proximal end connected to the remaining portion of the drilling tool. The proximal end of the drill bit is driven by a source of power such that the drill bit typically has a rotary motion, an axial percussive motion and/or some combination of these motions.
The proximal end of the drill bit is secured to the drilling tool for operation and is removable to allow use of a different drill bit or to service the current drill bit or the drilling tool. In drilling tools with bits capable of rotary and axial motion, there are multiple connections between the drill bit and the remaining portion of the drilling tool to allow torque and force to be transmitted to the drill bit.
In rock drilling applications, many of the drilling tools are operated by a pressurized fluid source and have an axial bore and openings in the drill bit through which fluid is conveyed. The axial bore and openings can convey a flushing medium out through the distal end of the drill bit. Alternatively, in a reverse circulation drilling tool, fluid and material can be drawn in through the openings and conveyed toward the proximal end of the drilling tool.
According to another conventional approach, although not widely used, a drill bit having a three-sided (i.e., non-circular) cross-section is secured in a surrounding chuck by a key extending transverse to the drill bit's axis, but the key does not extend through the center of the bit, and the bit must be provided with a flat section against which the key can engage. It is the three-sided shape of the chuck and bit, and not the key, through which torque is transmitted. The key fixes the drill bit's axial position relative to the chuck. In addition, the drill bit has a solid cross-section, and there is no provision for using the off-center transverse key in a drill bit having an axial bore.
According to another conventional approach, which is in wider use, the proximal end of the drill bit is received in a bore defined in a surrounding component, referred to as a driver sub. The bore and the drill bit have a first splined connection allowing torque to be transmitted to the drill bit. To secure the drill bit to the drilling, there is a second connection, such as by way of retaining rings, between a different portion of the drill bit and a surrounding component, such as a wear sleeve. The second connection allows axial force to be transmitted to the drill bit.
This conventional approach has several drawbacks. First, the splined connection is subject to excessive wear and early failure in some environments. The splined connection is also difficult to manufacture, making the drilling tool and drill bit more expensive. In addition, requiring two separate connections consumes additional space within the drilling tool and requires the drilling tool to have added length, which is a disadvantage. Moreover, the retaining rings used for the second connection can become difficult to remove after use.